Biomedical Engineering Reference
In-Depth Information
[20]
. These studies indicate that nanotubular, nanoporous TiO
2
structures can simulate the environ-
ment where bone formation/remodeling occur.
7.3
TIO
2
NANOTUBES FOR IMPLANT FABRICATION
Osteoblast adhesion and activity is enhanced on rougher titanium surfaces than on the smoother sur-
faces. Rougher surfaces enhance osteoblast activity as there is an increased surface area available for
cell (osteoblast) interaction. The studies in the past have focused on modifying titanium surface at
the microscale (10
6
m) and nanoscale (10
9
m) level. Therefore, fabrication of nanoscale structures
(nanotubes) on the titanium substrates will offer more surface area than the microscale rough surface.
Moreover, nanoscale features mimic the natural environment that the bone cells are accustomed to,
specifically, osteoblasts consistently interact with hydroxyapatite crystals (20-40 nm long) uniquely
patterned within a collagen matrix (type I collagen is a triple helix 300 nm in length, 0.5 nm in width,
and periodicity of 67 nm)
[21]
. Numerous studies are being conducted to refine the nanoporous tita-
nium substrate to ordered and controllable nanotubes with a surface that would closely mimic the
nanoarchitectural environment of human bone.
The first generation of the TiO
2
nanotube arrays was grown in hydrofluoric acid (HF) electro-
lytes or acidic HF mixtures (
Figure 7.2
)
[11]
. The thickness of the oxide layer was between 500 and
600 nm
[22,23]
. By substituting HF electrolytes with buffered neutral electrolytes containing sodium
fluoride (NaF) or ammonium fluoride (NH
4
F), self-organized TiO
2
nanotube layers with thicknesses
higher than 2 μm were grown
[24,25]
. The third-generation nanotubes were grown in (almost) water-
free electrolytes. Earlier studies in glycerol electrolytes showed tubes with extremely smooth walls
and tube length exceeding 7 μm
[26]
, while using acetic acid (CH
3
COOH) electrolytes
[27]
remark-
ably smaller tube diameters were obtained. In aged ethylene glycol electrolytes and by further opti-
mization of anodization parameters, the nanotube length reached 260 μm and the tubes with almost
ideal hexagonal arrangement were grown
[28]
. In general, the morphology and the structure of nano-
tubes were strongly influenced by the electrochemical conditions, particularly the anodization volt-
age and the solution parameters such as the HF concentration, the pH, and the water content in the
electrolyte).
Using the same approach of controlled anodization in dilute fluoride electrolytes, nanotubes of
intermetallic titanium compounds such as Ti-Al
[29]
, binary alloys such as TiNb
[30]
, TiZr
[31]
, or
on complex biomedical alloys such as Ti-6Al-7Nb
[32]
and Ti-29Nb-13Ta4.6Zr
[33]
were success-
fully fabricated (
Figure 7.3
). Attempts have also been made to grow self-organized anodic nanotube
layers on technologically relevant substrates such as Ti-6Al-4V and Ti-6Al-7Nb
[32,34]
for dental
and orthopedic implant applications.
7.4
FUNCTIONALIZATION OF TIO
2
NANOTUBES WITH GROWTH FACTORS
AND ANTIBACTERIAL/ANTI-INFLAMMATORY DRUGS
TiO
2
nanotubular structures can be functionalized with osteogenic (bone-producing) growth factors
which can play a vital role in implant osseointegration.
In-vivo
study in rats reported that the adhe-
sion of osteoblasts and bone regenerating ability were significantly accelerated by TiO
2
nanotubes
